With induction heating devices, a magnetic alternating field, which induces eddy currents in a cooking vessel which is to be heated and which has a bottom made of ferromagnetic material, is produced by means of an induction heating coil and causes losses due to reversal of magnetization, as a result of which the cooking vessel is heated.
The induction heating coil is part of a resonant circuit which comprises the induction heating coil and one or more capacitors. The induction heating coil is normally designed as a flat, helically wound coil with associated ferrite cores and is arranged, for example, under a glass ceramic surface of an induction hob. In doing so, the induction heating coil in conjunction with the cookware to be heated forms an inductive and a resistive part of the resonant circuit.
To drive or excite the resonant circuit, a low-frequency mains alternating voltage with a mains frequency of 50 Hz or 60 Hz for example is first rectified and then converted by means of semiconductor switches into an excitation or drive signal of higher frequency. The excitation signal or drive voltage is usually a rectangular voltage with a frequency in a range from 20 kHz to 50 kHz. A circuit to generate the excitation signal is also referred to as a (frequency) converter.
Different methods have been disclosed for adjusting a heating power supply to the cooking vessel depending on a set heating power setpoint.
In a first method, a frequency of the excitation signal or of the rectangular voltage is varied depending on the heating power to be emitted or supplied or on the required power transfer. This method for adjusting the heating power emission makes use of the fact that a maximum heating power emission occurs when the resonant circuit is excited at its resonant frequency. The greater the difference between the frequency of the excitation signal and the resonant frequency of the resonant circuit, the smaller the heating power emitted.
However, if the induction heating device has a plurality of resonant circuits, for example when the induction heating device forms an induction hob with different induction cooking zones, and different heating powers are set for the resonant circuits, beat frequencies, which can lead to annoying noises, can be caused due to superimposition of the different frequencies of the excitation signals.
A method for adjusting the heating power which prevents annoying noises due to beat frequencies of this kind is a pulse width modulation of the excitation signal at constant excitation frequency, with which an effective value of a heating power is adjusted by varying the pulse width of the excitation signal. However, with an effective-value control of this kind by varying the pulse width at constant excitation frequency, high switch-on and switch-off currents occur in the semiconductor switches, as a result of which a wide-bandwidth and energy-rich interference spectrum is produced.
It is frequently desirable to determine a temperature of the bottom of a cooking vessel which is inductively heated in this way in order, for example, to be able to generate specific time-dependent heating profiles, to determine a boiling point and/or to enable automatic cooking functions.
DE 10 2009 047 185 A1, which corresponds to pending U.S. Patent Application No. 2011/0120989, discloses a method and an induction heating device with which temperature-dependent ferromagnetic characteristics of the bottom of the cooking vessel are measured with high resolution and evaluated in order to determine the temperature of the bottom of the cooking vessel.
The characteristic of the temperature of the bottom of the cooking vessel when bringing foodstuffs, for example rice, floating in water to the boil behaves differently from when bringing pure water to the boil. Because the bottom of the pan is not completely covered with water but to a great extent with the foodstuff, convection in water is impeded. This makes the detection of the boiling point considerably more difficult.